Axial Compression Performance of Double Square Steel Tube Composite Confined Concrete Columns with Inner Oblique and Outer Straight Configuration

Literature Overview

The paper by Chen Zongping and Ning Fan, published in Industrial Construction (Vol. 52, No. 1, 2022, pp. 8-16), presents an experimental investigation into the axial compression behavior of a novel composite confined concrete column that employs a dual square steel tube system with an inner oblique tube and an outer straight tube. The research was supported by the National Natural Science Foundation of China (Grant 51578163) and several Guangxi provincial research programs. The study was conducted at the College of Civil Engineering and Architecture, Guangxi University, and the Key Laboratory of Engineering Disaster Prevention and Structural Safety (Ministry of Education). Eight composite column specimens were designed and tested alongside one conventional square steel tube concrete column (SC1) as a reference. The parameters varied included the inner square tube side length and wall thickness, outer square tube wall thickness, slenderness ratio (height-to-width ratio), and core column area ratio.

Core Technical Findings

The experimental results reveal several important mechanical behaviors of this dual-confinement system. The failure mode under axial compression is characterized by bending instability in the plastic stage combined with local buckling. The inner steel tube develops continuous folds under the combined action of the core concrete and the interstitial concrete, while the outer steel tube bulges outward due to concrete confinement pressure. Delamination regions between the inner and outer tubes exhibit varying degrees of outward bulging, and larger delamination areas significantly influence the ultimate failure morphology.

A key quantitative finding is that composite column specimens with a steel ratio comparable to specimen SC1 demonstrated a 35% increase in load-bearing capacity. The load-displacement curves of the composite specimens showed no descending branch, indicating excellent plastic strengthening characteristics. This is a notable improvement over conventional single-tube steel tube concrete columns, which typically exhibit a descending branch after peak load.

Interpretation of Technical Points

The dual-confinement mechanism is the central innovation of this study. The inner oblique tube and outer straight tube together provide a composite confinement effect on the concrete core. The oblique orientation of the inner tube is particularly noteworthy, as it introduces a geometric interaction that differs fundamentally from conventional concentric dual-tube systems. The interstitial concrete between the two tubes plays a critical role in transferring confinement forces and maintaining composite action.

The absence of a descending branch in the load-displacement curves is attributed to the progressive engagement of confinement mechanisms. As the inner tube folds, it continues to exert lateral pressure on the core concrete, while the outer tube resists outward bulging. This sequential failure mechanism provides a natural plastic reserve that prevents sudden load capacity loss.

The observation that increasing wall thickness did not proportionally increase peak load is an important practical insight. This is partly attributed to uneven concrete mixing, which creates heterogeneous concrete properties within the confined volume. However, increased wall thickness did improve post-peak performance, suggesting that thicker tubes provide better ductility and energy dissipation capacity even when the peak strength is not enhanced.

The slenderness ratio effect is particularly relevant for practical design. When the height-to-width ratio reaches 6, second-order effects become significant, altering the failure mode and degrading mechanical indicators. This establishes a practical upper limit for the applicability of this composite column system under pure axial compression conditions.

Standards and Design Methodology Analysis

The paper proposes a calculation method that comprehensively considers the confinement effects of both the inner and outer square steel tubes. The calculated values showed good agreement with experimental measurements, validating the proposed analytical approach. This is significant because existing design codes such as GB 50017, CECS 28, and AISC specifications primarily address single-tube steel tube concrete members and do not adequately capture the composite confinement behavior of dual-tube systems.

The steel ratio concept used in this study follows the convention established in Chinese steel tube concrete design codes, where the steel ratio is defined as the ratio of the cross-sectional area of steel to the total cross-sectional area of the member. The finding that increasing the core column area ratio at a similar steel ratio better exploits the material advantages of both steel and concrete has direct implications for optimizing the geometric design of composite confined columns.

Engineering Practice Implications

From a practical engineering perspective, this dual-confinement composite column system offers several advantages for seismic-resistant structures and high-rise buildings. The enhanced ductility and energy dissipation capacity, evidenced by the absence of a descending branch and the excellent plastic strengthening behavior, make this system particularly suitable for applications requiring high deformation capacity under cyclic loading.

However, several practical challenges must be addressed before widespread adoption. The construction of the inner oblique tube within a square outer tube requires careful fabrication and assembly procedures. The delamination between the inner and outer tubes, while not catastrophic, represents a potential weakness that could be exacerbated by poor construction practices or inadequate concrete placement quality. The interstitial concrete between the two tubes is difficult to compact thoroughly, and uneven concrete mixing was identified as a factor affecting peak load performance.

The 35% increase in load-bearing capacity at comparable steel ratios is a compelling economic argument for this system. In applications where space is constrained and high load capacity is required, this composite column could offer a more efficient use of materials compared to conventional single-tube solutions.

Key Questions and Reflections

Several questions arise from this study that warrant further investigation. First, the study focuses exclusively on axial compression, and the behavior of this dual-confinement system under eccentric loading, bending, and combined loading conditions remains unknown. Given that real structural columns are rarely subjected to pure axial compression, this is a significant limitation. Second, the influence of the oblique angle of the inner tube on the confinement effectiveness was not systematically investigated. Third, the long-term behavior under sustained loads, including creep and shrinkage effects, was not addressed. Fourth, the seismic performance under cyclic loading, which is critical for earthquake-resistant design, was not evaluated in this study.

The construction quality issues identified, particularly uneven concrete mixing, suggest that rigorous quality control measures are essential for achieving the full potential of this system. From a welding and fabrication standpoint, the connections between the inner and outer tubes, and the detailing at the column ends, would require careful attention to ensure composite action throughout the member length.

Summary

This study presents a promising novel composite column system that leverages dual steel tube confinement to significantly enhance axial compression performance. The 35% improvement in load-bearing capacity at comparable steel ratios, combined with excellent ductility and plastic strengthening behavior, demonstrates the technical viability of this approach. The proposed calculation method, validated against experimental data, provides a foundation for design applications. However, the system's behavior under more realistic loading conditions, the optimal oblique angle of the inner tube, and long-term durability considerations remain areas requiring further research. For engineering practice, this dual-confinement concept represents an innovative direction that could contribute to more efficient and resilient structural systems, particularly in seismic zones where ductility and energy dissipation are paramount design objectives.